Due to the advance in a communication system, a wide variety of wireless devices have been introduced to consumers such as businesses and individuals.
A current mobile communication system has been affiliated with third generation partnership project (3GPP), for example, Long Term Evolution (LTE), LTE-Advanced, the like. Such a mobile communication system is a high-speed large-capacity communication system capable of transmitting and receiving various data such as video and wireless data beyond a voice-oriented service.
The high-speed large-capacity communication system requires a technology for improving the capacity of a UE by using a small cell. That is, it is required to develop a technique for providing radio resources to one UE from two or more base stations so as to process the high-speed large-capacity data.
On the other hand, in order to configure additional radio resources by the UE while performing communication through one base station, it requires a procedure for adding radio resources of other base stations and starting communication.
In addition, when a problem occurs in the procedure for adding radio resources of other base stations and starting communication using the added radio resources, it also requires a method for handling such a problem and a specific procedure for fast communication recovery by the UE.